A fuel cell is a device that converts the chemical energy of fuel and an oxidant directly into electricity without combustion. Fuel cells are considered superior to processes that involve the burning of fuels because fuel cells have higher conversion efficiency and in general produce little to no air pollutants.
The components of the fuel cell include electrodes that are catalytically activated for fuel (anode) and the oxidant (cathode), and one or more electrolytes that conduct ions between the electrodes, thereby producing electricity. The electrodes are incorporated with catalysts to increase the reaction rates such that the energy conversion can proceed at an acceptable rate.
Fuel cells use fuel in the form of hydrocarbons (such as alcohols and hydrocarbons including gasoline and diesel) or, more commonly, hydrogen. Fuel cells typically use oxidant that is in the form of oxygen in the air.
Unlike internal combustion engines, fuel cells are not limited by the Carnot cycle and thus, in principle, could reach higher efficiencies. With pure hydrogen as the fuel, fuel cells are also environmentally friendly. The combination of high efficiency, low environmental impact, and higher power density has been and will continue to be the driving force for active research in this area for a wide variety of applications such as transportation, residential power generation, and portable electronic applications. For portable electronic applications in particular, desirable features include high energy density (i.e., longer battery life) and compactness.
There are many types of fuel cells. One example is the polymer ion exchange membrane fuel cell (IEMFC), which further includes the proton exchange membrane fuel cell (PEMFC), the anion/hydroxide exchange membrane fuel cell (AEMFC/HEMFC), and the bipolar membrane fuel cell (BMFC). When methanol is used directly as the fuel in these types of cells, the fuel cell is often referred to as a direct methanol fuel cell (DMFC). For these fuel cell examples, the electrolyte is typically based on membranes made from charged polymers (ionomers).
For PEMFC and PEMFC-based DMFCs, protons are the mobile species that travel through the ionomer electrolyte from the anode toward the cathode. Nafion is an example of a robust ionomer commonly used in PEMFC and DMFC. Illustrative ionomers are described in U.S. Pat. No. 7,829,620, titled “Polymer-Zeolite Nanocomposite Membranes for Proton-Exchange-Membrane Fuel Cells,” filed on Sep. 25, 2009, which is incorporated by reference herein in its entirety.
For AEMFC/HEMFC types of cells, a hydroxide ion species resulting from oxygen reduction at the cathode is the mobile species that travels through the ionomer electrolyte from the cathode toward the anode.
Anion/hydroxide exchange membrane fuel cells (AEMFCs/HEMFCs) have received increasing attention due to perceived advantages such as (a) more facile fuel oxidation and oxygen reduction in high pH media, (b) electro osmotic drag by OH− from cathode to anode, which not only reduces fuel crossover but also realizes anode drainage, and (c) elimination of the bi/carbonate contamination problem of traditional liquid alkaline fuel cells (AFCs) whose electrolyte contains free metal cations. See, e.g., C. Lamy, E. M. Belgsir, J. M. Leger, Journal of Applied Electrochemistry 31, 799 (2001); Y. Wang, L. Li, L. Hu et al., Electrochemistry Communications 5 (8), 662 (2003); J. R. Varcoe and R. C. T. Slade, Fuel Cells 5 (2), 187 (2005).
A suitable anion/hydroxide exchange ionomer will likely have three-phase boundaries at the electrodes, where catalysts, electrolyte and reactant can interact. Unfortunately, unlike high performance acidic Nafion ionomer for proton exchange membrane fuel cells (PEMFCs), comparable hydroxide exchange ionomer has not heretofore been available for AEMFCs/HEMFCs.
KOH or NaOH aqueous solution has sometimes been used in the electrodes as an exchange ionomer, which limits advantages of AEMFCs/HEMFCs over traditional AFCs. See, e.g., K. Matsuoka, Y. Iriyama, T. Abe et al., Journal of Power Sources 150, 27 (2005); E. H. Yu and K. Scott, Journal of Power Sources 137 (2), 248 (2004); E. Agel, J. Bouet, and J. F. Fauvarque, Journal of Power Sources 101 (2), 267 (2001); L. Li and Y. X. Wang, Journal of Membrane Science 262 (1-2), 1 (2005); C. Coutanceau, L. Demarconnay, C. Lamy et al., Journal of Power Sources 156 (1), 14 (2006). Non-ionic conductive PTFE has also been used as an ionomer, which does not provide OH− transfer in the electrode and thus does not perform satisfactorily. See, e.g., E. H. Yu and K. Scott, Journal of Applied Electrochemistry 35 (1), 91 (2005). Sometimes Nafion is used as an ionomer, which also restrains OH− transfer in the electrode dramatically and is thus it not satisfactory either. See, e.g., H. Y. Hou, G. Q. Sun, R. H. He et al., Journal of Power Sources 182 (1), 95 (2008); A. Verma and S. Basu, Journal of Power Sources 174 (1), 180 (2007).
One approach is to crosslink membrane polymers in an attempt to alter the ionic conductivity. Crosslinking of membrane polymers and its effect on ionic conductivity in both PEMs and HEMs have been reported. See Z. L. Zhou, R. N. Dominey, J. P. Rolland, B. W. Maynor, A. A. Pandya and J. M. DeSimone, Journal of the American Chemical Society, 2006, 128, 12963-12972; 12; N. J. Robertson, H. A. Kostalik, T. J. Clark, P. F. Mutolo, H. D. Abruna and G. W. Coates, Journal of the American Chemical Society, 2010, 132, 3400-3404. However, one of the undesirable effects of crosslinking the ionomer is that the membrane tends to become less flexible, sometime even brittle, due to the fixed polymer chains. It would be advantageous to provide a fuel cell in which crosslinking and flexibility are balanced.
Recently, an insoluble cross-linked di-amine quaternized polyvinyl benzyl electrochemical interface was prepared as an attempt to enhance HEMFC performance. See, e.g., J. R. Varcoe, R. C. T. Slade, and E. Lam How Yee, Chemical Communications (13), 1428 (2006); J. R. Varcoe and R. C. T. Slade, Electrochemistry Communications 8 (5), 839 (2006). This polymer is not a soluble ionomer, however, and cannot effectively build three-phase boundaries in electrodes. As a result, its performance in HEMFCs is still limited. In addition, its ionic conductivity and stability are also limited because of its quaternary ammonium hydroxide group. Also recently, a soluble alkaline ionomer, A3-solution produced by Tokuyama was reported; however, its chemical structure, preparation method, and material properties such as ionic conductivity, stability, and fuel cell performance, are unknown. See, e.g., H. Bunazawa and Y. Yamazaki, Journal of Power Sources 182 (1), 48 (2008).